1. Field of the Invention
This invention relates broadly to Stirling engines. More particularly, the invention relates to a Stirling engine having a fluid heat exchanger adapted to have improved heat transfer and operate under high pressure and temperature.
2. State of the Art
Frequently heat energy must be exchanged between two or more fluids which do not mix and which may be flowing or stagnant. The heat energy is transferred from the hotter fluid to a separating wall by convection and/or radiation. Heat energy is conducted through the wall from the hot side to the cold side. Heat energy is then transferred from the separating wall to the cooler fluid by convection and/or radiation. The purpose of the heat exchanger may be to raise the temperature of a relatively cool fluid (as a heater) or to lower the temperature of a relatively hot fluid (as a cooler).
Except for radiative only heat exchangers, all heat exchangers have large surfaces where heat energy is absorbed or given off by the surface contacted by the fluids. There are basically three types of fluid heat exchangers for Stirling engines defined by the fluid interfacing configurations. Heat exchangers for Stirling engines may be annular, finned, or tubular, or various combinations of these. Annular heat exchangers consist of concentric tubes with the fluids contained in or between them. The tubes may be cylindrical or of other closed cross sections. One tube separates the fluids and provides the surface area and conductive path required for heat exchange. Finned heat exchangers increase the surface area exposed to one or both fluids by providing finned structures on one or both sides of the wall, which effectively increase the surface area of the wall thus improving heat transfer. Tubular heat exchangers contain one fluid within relatively small diameter tubes that are surrounded by the other fluid. Heat is conducted through the tube wall. Various combinations of these three types may also be used in a heat exchanger. For example, fins may be added to the tubes of an annular heat exchanger to increase the contacted surface area.
Annular (with and without fins) and tubular heat exchangers have been used for Stirling engines. Tubular heat exchangers (with and without fins) have been traditionally used for engines with power outputs greater than 1 kW mechanical. Many small diameter tubes provide large surface area and the small diameters have lower stress at high pressures. Tubular heat exchangers are the most expensive to produce and are susceptible to burnout due to uneven heating and high stresses at the attachment points due to thermal expansion deformation of long tubes.
Often one or more of the fluids may be pressurized to a relatively high level. In such case, the separating wall must structurally resist the difference in pressure between the fluids. For high heat exchanger efficiency, large fluid contacted surfaces and low thermal resistance through the separating wall are desired. Low thermal resistance is achieved by using a thin separating wall, large contact area, and a material with high thermal conductivity. On the other hand, high structural strength to resist deformation by pressure is achieved by using thick walls, small surface areas, and high strength materials. In general materials with high thermal conductivity do not have high strength and high strength materials have low thermal conductivity. Thus, the desired characteristics of heat exchanger designs assuring high thermal efficiency and high strength conflict.
With particular reference to Stirling engines, such engines are typically provided with four heat exchangers: a heater, a regenerator, a cooler, and an exhaust/inlet air preheater. A more detailed explanation of the respective functions of the heat exchangers of Stirling engines can be found in G. Walker in xe2x80x9cStirling Enginesxe2x80x9d, Clarendon Press, 1980, pp. 124-126, 133-144, and 156-159, which is hereby incorporated by reference herein in its entirety. The above described annular, tubular, and finned heat exchangers, as well as combinations thereof, have all been used in various Stirling engines for heaters and coolers. For example, U.S. Pat. No. 4,671,064, which is hereby incorporated by reference herein in its entirety, describes an annular heat exchanger for a Stirling engine. C. M. Hargreaves in xe2x80x9cThe Philips Stirling Enginexe2x80x9d, Elsevier, 1991, pp. 185-187, describes finned heat exchangers (referred to as xe2x80x9cconcertinaxe2x80x9d and xe2x80x9cpartitionxe2x80x9d heaters) in Stirling engines.
For maximum efficiency, the Stirling engine working fluid temperature should be as high (as close to the heating fluid temperature) as possible at the heater and as low (as close to the cooling fluid temperature) at the cooler as possible. For maximum power production, the working fluid pressure should be as high as possible. This requires high thermal conductivity of the wall separating the fluids and high strength at the operating temperature. Heating fluid temperature should be as high as the heat exchanger construction material can withstand at the working fluid pressure.
One manner of increasing the pressure-resisting strength of a pressure vessel is to use xe2x80x9corthogonal grillagexe2x80x9d about a separating wall; i.e., providing straight internal fins parallel to the cylinder axis combined with disk-like external fins perpendicular to the axis and integral to the separating wall. The straight and disk-like fins cross each other at right angles. xe2x80x9cOrthogonal grillagexe2x80x9d is described in more detail in J. F. Harvey in xe2x80x9cTheory and Design of Modern Pressure Vesselsxe2x80x9d, 2nd Ed., Van Norstrand Reinhold, 1974, pp. 120-122, which is hereby incorporated by reference herein in its entirety. However, orthogonal grillage has the disadvantage in that it is complicated and difficult to move a heating fluid around the pressure vessel to permit the heat exchange.
It is therefore an object of the invention to provide a heat exchanger for heating or cooling a fluid in a high pressure vessel.
It is another object of the invention to provide a heat exchanger which has a relatively high structural integrity.
It is a further object of the invention to provide a heat exchanger through which it is relatively easy to circulate heating fluid.
It is an additional object of the invention to provide a heat exchanger which has a high heat transfer efficiency.
It is also an object of the invention to provide a heat exchanger which is relatively light weight.
It is still another object of the invention to provide a heat exchanger which is relatively inexpensive to manufacture.
It is yet another object of the invention to provide a heat exchanger for a Stirling engine.
In accord with these objects, which will be discussed in detail below, an annular heat exchanger having helical fins is provided. According to preferred aspect of the invention, an outer reinforcing sleeve is provided about the helical fins. The sleeve improves the pressure resisting ability of a thin separating wall (e.g., the heater wall of a Stirling engine) resulting in a high-pressure heat exchanger with high heat transfer efficiency. In addition, the sleeve and helical fins together define fluid passages for the flow of a heating fluid.
The heat exchanger according to the invention has an ability to resist high pressures at high temperatures without excessive or permanent distortion, has an improved heat transfer capability, better reliability, and lower production cost than prior art heat exchangers.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.